1. Field of the Invention
The present invention relates to an optical near-field generating device capable of generating optical near-fields by irradiating a scatterer made of a conductive material with light, an optical near-field generating method and an information recording and reproducing apparatus.
2. Description of the Related Art
As high density recording of magnetic recording technology advances more in recent years, development of a recording system for recording a magnetic recording film with high coercive force capable of high density recording is requested. As a promising method for developing such recording system, a heat-assisted magnetic recording (or light-assisted magnetic recording) in which a recording area is locally irradiated with light to locally lower coercive force of a magnetic recording medium to enable a magnetic head to record information receives a remarkable attention. In order to obtain high density magnetic recording in the heat-assisted magnetic recording, it may be necessary to reduce a size of a beam spot of focusing light. As a method of obtaining a small focusing beam spot beyond a limit of diffraction of light, various technologies using optical near-fields have been proposed and a method using surface plasmons generated from a metal scatterer, for example, is known as one of such proposed technologies. When a metal scatterer is in use, a shape of a scatterer affects a size of a spot of an optical near-field and focusing efficiency of the optical near-field. Therefore, various studies have been made so far on the shape of practical scatterers capable of generating optical near-fields with high efficiency.
A method of obtaining a small focusing beam spot by using surface plasmon resonance phenomenon caused by a metal scatterer will be described with reference to FIG. 1. As shown in FIG. 1, a rod-like scatterer 410 made of a conductive metal is formed on the flat surface of a substrate 401 typically made of a light transmissive material. The longitudinal direction of the scatterer 410 and the polarization direction of irradiated light are aligned and the length in the longitudinal direction of the scatterer 410 is suitably selected in accordance with conditions under which surface plasmons may be excited, thereby exciting surface plasmons.
The scatterer 410 located and configured in accordance with such suitable conditions is irradiated with light Li from the side of the substrate 410, as shown in FIG. 2. As a result, charges are polarized by an electric field caused by the incident light Li on a light-receiving surface 410d serving as the surface of the scatterer 410 irradiated with the incident light L1 and a light-emitting surface 410e of the opposite surface of the light-receiving surface 410d and which is the surface facing an object 450 to optical near-fields. Oscillation generated when charges are polarized is surface plasmons. When a resonance wavelength of surface plasmons is equal to a wavelength of incident light Li, surface plasmons are in a resonance state called surface plasmon resonance and the scatterer 410 becomes an electric dipole, which is strongly polarized in the direction of polarization. When the scatterer 410 becomes the electric dipole, large electromagnetic field is generated near respective ends in the longitudinal direction of the scatterer 410 to generate an optical near-field Ln. As shown in FIG. 2, while the optical near-field Ln is generated on both the light-receiving surface 410d and the light-emitting surface 410e of the scatterer 410, their optimum resonance wavelengths may vary depending on materials and shapes of structural bodies around the scatterer 410. When applying an optical near-field to the object 450 such as an information recording medium, the shape of the scatterer 410 should be adjusted so that the optical near-field Ln on the light-emitting surface 410e may be intensified.
In the thus adjusted scatterer 410, an intensive optical near-field can be generated with a beam spot of a small diameter. Since the scatterer 410 becomes the electric dipole, an optical near-field is generated in two places and there is such a problem that optical near-fields may be applied to other portions than required. In a heat-assisted magnetic recording, when an optical near-field is applied to other portions than required, if a mark indicating that information was already magnetically recorded exists on such portions, then a recording retention life of the object such as an information recording medium is reduced by thermal demagnetization, which affects reliability of the information recording medium considerably.
Japanese Unexamined Patent Application Publication No. 2004-151046 (JP 2004-151046 A), for example, proposes a method of removing the surface of other portions than a vertex to generate an optical near-field by a treatment such as etching so that a depth between an object and the other portions is deeper than a reaching depth of the optical near field.
Also, Japanese Unexamined Patent Application Publication No. 2004-303299 (JP 2004-303299 A), for example, proposes a method of improving integration of a magnetic head and an optical head, in which an optical near-field generating portion and a magnetic field generating portion to be the same position by providing a narrowed portion in a conductor.